1. Technical Field
The invention generally relates to motor control systems for controlling the operation of AC motors.
2. Description of the Background Art
A typical three-phase AC motor control system includes a power circuit and a control circuit. The power circuit usually includes an AC-to-DC converter, an intermediate DC circuit and an inverter for reconverting DC power to alternating currents that are supplied to the stator of the motor. The inverter includes network of at least six semiconductor switches, which may be thyristors or power transistors. The phase currents in the stator of the motor may be precisely controlled by controlling the firing or conduction of the semiconductor switches with one of several known control circuits. One type of control circuit uses a pulse width modulation (PWM) to control the operation of the semiconductor switches.
A typical three-phase PWM motor drive generates a pulse width modulated (PWM) signal to the power semiconductors in a DC-to-AC inverter. The PWM generator circuit is often operated in a sine-triangle mode in which a time-varying sinusoidal command signal is compared with a triangular waveform and in which the crossing points of the two waveforms define the rising and falling edges of the pulses generated in the resulting PWM pulse train.
Another known type of modulating circuit used in motor control is referred to as a hysteresis modulator. With this type of modulator a command signal is compared to an upper reference and a lower reference to limit the resulting signal within a hysteresis band having an upper limit and a lower limit.
The following is a summary of background patents which are cited to show examples of techniques used in the art of PWM motor drives.
Sakamoto et al, U.S. Pat. No. 4,458,192, FIG. 2, provides one example of current limiting circuitry in a 3-phase PWM drive. The object of this current limiting is to prevent saturation of the current amplifiers when the 3-phase currents are unbalanced.
Okado et al., No. 4,364,109 shows a PWM inverter which is controlled in the hysteresis mode and in the sine-triangle mode. In this patent the circuitry for the two modes is connected in parallel, and the switching between the two modes is based on frequency range rather than on magnitude.
Opal et al., U.S. Pat. No. 3,819,992 et al, FIG. 1, shows a current limiting circuit for controlling an input to a PWM modulator to provide both upper and lower voltage limits from being exceeded, however, the response is again based on frequency rather than on magnitude of current.
Plunket, U.S. Pat. No. 4,047,083 shows a PWM drive that operates in the traditional sine-triangle mode and in a second mode.
Riesco, No. 4,672,521, shows a hysteresis-type circuit for limiting the magnitude of a sine wave reference signal within a certain band (see FIG. 5). Okuyama, No. 4,486,824 shows a hysteresis-type PWM inverter in FIG. 5.
Sekino et al., No. 4,646,221, and Azusawa et al., No. 4,628,475 show sine-triangle PWM inverter circuits.
Sine-triangle modulators are known to run at a frequency that is limited and controlled by the triangle wave generator which normally prevents the system from migrating to system resonant frequencies and causing instability in closed loop systems. Also, current ripple and torque disturbances are normally low in the steady-state as compared to other techniques.
A technical problem may arise, however, in closed loop PWM motor drive systems during transient conditions which cause large undesirable instantaneous values for current in the PWM-generated wave. This may limit the reliability or range of operation of the motor drive. One such transient condition may occur when power is interrupted and then reapplied to a motor that is still rotating. Under this condition transient currents may become too great in a sine-triangle PWM modulating circuit.